Aluminum is produced industrially by igneous electrolysis, using the well-known Hall-Héroult process, in electrolysis cells. The plants contain a great number of electrolysis cells laid out in line, in buildings called electrolysis halls or rooms, and electrically connected in series using connecting conductors, in order to make the best use of the floor area of the plants. The cells are generally laid out so as to form two or more parallel lines which are electrically linked to each other by end conductors. In each cell, the electrolyte bath and the molten metal are contained in tanks, called “electrolysis tanks”, comprising a steel container, which is coated on the inside with refractory and/or insulating materials, and a cathodic unit located at the bottom of the tank. Anodes, typically made of carbonaceous material, are partially immersed in the electrolyte bath.
When operating, an electrolysis plant requires work on the electrolysis cells, including replacement of worn anodes by new ones, sampling of molten metal in the cells and sampling or top-ups of electrolyte. In order to carry out this work, the most modern plants are generally equipped with one or more service units including an overhead traveling crane which can be relocated above the electrolysis cells, along series of cells, and one or more service machines each including a carriage able to be moved on the overhead traveling crane, and a service module provided with handling and servicing devices such as shovels and hoists, commonly known as “tools”. These service units are often called “Pot Tending Assemblies” (PTA) or “Pot Tending Machines” (PTM). The service module generally includes a tool-holder turret, each tool either being fixed at the end of a cable operated by a winch attached to said tool-holder turret, or fixed at the end of a mobile, typically telescopic or articulated arm, the other end of which is attached to said tool-holder turret.
One of the operations necessary during the anode replacement is the cleaning of the part of the liquid medium made up by the bath and the molten metal which was covered by the worn anode and which must be covered by the new anode. During electrolysis, a hard crust of fluorinated cryolite and alumina is formed on the upper surface of the bath. This crust has the advantage of storing the heat within the bath and therefore provides an effective insulating envelope. But it is extremely hard and adheres to the wall of the anode block, so that it proves to be necessary to break it around the worn anode, in order for the latter to be extracted. Typically, the crust is broken up out using tools such as tappers, called “crustbreakers”. During removal of the worn anode, there then forms an opening in the crust, which is left until the new anode is fitted and which we will thereafter refer to as the “anode hole”. Breaking up the crust and handling the worn anode block inevitably generate the formation of solid pieces or parts which float on, or remain in suspension in the liquid medium made up by the electrolyte bath and the molten metal, or which fall to the bottom of the tank. It turns out to be necessary to remove these solid parts by means of a collecting tool, commonly known as a “crust shovel”. In addition, mud settles gradually to the bottom of the tank, i.e. on the cathode, forming an increasingly thick layer, which increases resistance and consequently reduces the output of said tank. The crust shovel in the anode hole is therefore also used to also remove this mud which has accumulated on the bottom of the tank.
European patent application EP-A-0 440 488 described an example of a crust shovel in conjunction with a particular vehicle, as distinct from a service machine. European patent application EP-A-0 618 313 described in a far from detailed way an example of a service machine equipped with a device able to break up the crust in the vicinity of a worn anode and to clean the anode hole. Whether it is conveyed by a special vehicle or a service machine, the crust shovel commonly used takes the form of a grip made up of two buckets placed in a substantially symmetrical way in relation to a substantially vertical plane and articulated, swiveling around two substantially horizontal axes, that may be one and the same. Each bucket has a leading edge, also called a “blade”, opposite the leading edge of the other shovel. To collect the remains, the crust shovel, in open position, is plunged into the bath, and then the crust shovel is moved from an open position to a closed position, by using at least one actuator which works either directly on a bucket, or on a connecting rod assembly designed to make the buckets rotate substantially symmetrically in relation to each other, the solid remains located between the two buckets being therefore trapped, while the liquid medium, a mixture electrolyte bath and molten metal, can still escape, in particular through openings worked into the walls of the buckets. Patent application EP 1 178 004 describes such a bucket shovel, with a special duplicated frame.
The conditions in which the shovel is used, especially its buckets, designed to plunge into the liquid medium made up by the liquid bath floating on top of the molten metal, are particularly demanding, from both the heat and the mechanical standpoints, requiring frequent repairing of said bucket shovel. In general, this repair work requires replacement of the buckets, which takes several hours and requires the whole device, namely of the vehicle or the service machine provided with all its tools, to be shut down.